Air-conditioning system that operate on the basis of mass and heat exchange over so-called membrane contactors receive attention in an attempt to improve efficiency. Basically, such systems comprise a dryer, a humidifier and a regenerator. Schematic diagrams hereof look rather simple, though the understanding of the operation is rather complex. In fact, such systems are multi-phase and multi-chamber reactor in which mass and heat transfer occurs. An additional complicating factor resides in the desiccant flow: if the solution of desiccant becomes too concentrated, crystals may develop which may reduce the mass transfer in the membrane contactor or even obstruct the desiccant flow. However, relatively high concentrations of desiccant are desired for efficient operation of the system. A further complicating factor resides therein that different locations in the world require significantly different air treatments, so as to arrive in a so-called comfort zone that is appreciated by most people.
One example of an air-conditioning system is known from U.S. Pat. No. 6,887,302. This document proposes the use of a metering device configured to add water so as to increase the water content of the desiccant flow, as well as a control device therefor. By means of adding water to the desiccant flow, the operation of the membrane contactor could be modified, so as to switch between humidification en dehumidification. This known solution however is disadvantageous energetically, as it is very hard to remove again water from the desiccant flow once needed. Moreover, the addition of water is effectively a thinning of the desiccant flow, but it is not evident to obtain a homogeneous solution quickly.
Another example of an air-conditioning system is known from WO2009/094032A1. That prior document discloses a module design wherein flow of cooling fluid, desiccant flow and air flow are integrated into a single multilevel module. As shown in FIG. 1, the air flow (inlet airstream) runs in parallel to the liquid desiccant flow. This reduces the overall both heat and mass transfer efficiency relative to a countercurrent flow design.
A further example of an air-conditioning system is known from manuscripts of Dr Manuel Conde-Petit, particularly ‘Open absorption systems for air-conditioning using membrane contactors’, 15. Schweizerisches Status-Seminar<<Energie-and Umweltforschung im Bauwesen>>, 11-12 Sep. 2008 in Zurich, and ‘Open absorption system for cooling and air conditioning using membrane contactors, 2006 Annual report’ for the Swiss Government. For sake of clarity, these publications will be referred to as the 2008 report and the 2006 report respectively.
Conde herein proposes a modular design. FIGS. 12 and 13 of the 2008 report show the system that has been designed, made and tested. FIG. 3 of the 2008 report shows a more elaborate diagram. As is clear from FIG. 12, the system is provided with a first air inlet for the air to be treated and a second air inlet for use as a gas flow in the regenerator. The air from the regenerator is removed as an air exhaust. The regenerator unit further comprises a heat exhanger, wherein inlet air is cooled relative to the exhaust air that has passed the regenerator. Both the dryer, referred to as ABS or absorber, and the regenerator, referred to as GEN or desorber, are operated as three-stream contactors of air flow, desiccant flow and water. The 2008 report thereto states that it was clear ‘from the beginning that a continuously cooled absorber and a continuously heated desorber would be necessary in the prototype to be built’.
The three-stream membrane contactor is shown in more detail in the 2006 report. It is built out of five structured plates that may be of equal or different size (cross-section), where the membrane constitutes the interface between the air channels and the solution channels. The air flow and the water flow run in a countercurrent manner in parallel channels. The desiccant flow runs in channel extending substantially perpendicular to those for the air flow and the water flow. The structured plates are made of polymer material and are modified by milling, after having been cut to the right size. Due to the small wall thickness of the plates (0.1-0.3 mm), this was delicate according to the 2006 report. Thereto the structured plates were filled with water and freezed on top of a thermally conducting metallic plate. The frozen plate was thereafter milled, using the metallic plate as a carrier. The delicate multi-step manufacturing is a clear disadvantage of the modules used by Conde. It is therefore desired to obtain an improved air-conditioning system with modules that may be made more easily, but still meeting the requirements of the overall process in operation. In fact, said modules were designed by Conde for optimum mass and heat transfer, as deemed necessary to achieve the continuous cooling or heating as deemed necessary.